1. Field of the Invention
The present invention concerns a positive type resist composition for use in the step of manufacturing a semiconductors such as IC, and lithographic steps for the manufacture of circuit substrates, for example, in liquid crystals and thermal heads and other photo-applications, as well as a method of forming a pattern using the same. Particularly, it relates to a positive type resist composition suitable to exposure by a liquid immersion type projection exposure systems using far UV-light at a wavelength of 300 nm or less as a light source, as well as a method of forming the pattern using the same.
2. Description of the Related Art
Along with refinement of semiconductor devices, development has been conducted for making the wavelength of a exposure light source shorter and making the numerical aperture of a projection lens higher (high NA), and a developing apparatus having NA of 0.84 using, as a light source, an ArF excimer laser having a wavelength at 193 nm has been developed at present. The resolution and the focal depth can be represented by the following equations.(Resolution)=k1·(λ/NA)(Focal depth)=±k2·λ/NA2,where λ is wavelength of an exposure light source, NA is a numeral aperture of a projection lens, and k1 and k2 are coefficients related to a process.
An exposure apparatus using an F2 excimer laser having a wavelength at 157 nm as a light source has now under investigation for attaining higher resolution by making the wavelength further shorter but since the lens material used for the exposure apparatus and the material used for the resist are extremely restricted for shorting the wavelength, reduction of the manufacturing cost for the apparatus and the material and for stabilization of quality are extremely difficult, which have resulted in the possibility that exposure apparatus and resist having sufficient performance and stability can not be in time for the requested term.
As a technique for improving the resolution of an optical microscope, a so-called liquid immersion method has been known so far of filling a liquid of high refractive index (hereinafter also referred to as “liquid immersion solution”) between a projection lens and a specimen.
For the “effect of liquid immersion”, the resolution and the focal depth described above can be represented by the following equations in a case of liquid immersion assuming the wavelength of exposure light in air as λ0, the reflective index of the liquid immersion solution to air as n, a conversing semi-angle of a light as θ, and NA0=sin θ:(Resolution)=k1·(λ/n)/NA0 (Focal depth)=±k2·(λ/n)/NA02 
That is, the effect of the liquid immersion is equivalent with that in the case of using 1/n exposure wavelength. In other words, the focal depth can be increased to n times by liquid immersion in a case of a projection optical system of identical NA.
This is effective to all sorts of pattern shapes and, further, can be combined with super resolution technique such as a phase shift method, a modified illumination method, etc. which have been under investigation at present.
Examples of apparatus applying the effects described above to the transfer of fine patterns of semiconductor devices are described in JP-A No. 57-153433, JP-A No. 7-220990, etc., but they do not described resists suitable to the liquid immersion technique.
JP-A No. 10-303114 points out that control for the refractive index of the liquid immersion solution is important since the change of the refractive index of the liquid immersion solution causes degradation of projected images due to spherical aberration of exposure apparatus and discloses control of the temperature coefficient of the refractive index of the liquid immersion solution to a certain range, and water with addition of additives for lowering the surface tension or increasing the surface activity as a suitable liquid immersion solution. However, disclosure of the additives or the resist suitable to the liquid immersion exposure technique is not discussed.
Development of the recent liquid immersion exposure technique is reported, for example, in Bulletin of the International Society for Optical Engineering (Proc. SPIE), 2002, Vol. 4688, p 11, and J. Vac. Sci. Tecnol. B, 17 (1999), etc. In a case of using an ArF excimer laser as a light source, it is considered that pure water (refractive index at 193 nm of 1.44) is considered most prospective as a liquid immersion solution with a view point of handling safety, and transmittance and refractive index at 193 nm.
In a case of using an F2 excimer laser as a light source, while a solution containing fluorine has been considered in view of the balance between the transmittance and the refractive index at 157 nm, no satisfactory solution in view of the circumstantial safety and the refractive index has yet been found. In view of the degree of the effect of liquid immersion and the degree of completion of the resist, it is considered that the liquid immersion exposure technique will be adopted at first to the ArF exposure apparatus.
Since the resist for use in a KrF excimer laser (248 nm), an image forming method of chemical amplification is used as a method of forming resist images in order to compensate the lowering of sensitivity caused by light absorption. Referring to the example of the image forming method of the positive type chemical amplification, this is an image forming method of decomposing an acid generator in an exposed area by exposure to generate an acid, converting the alkali insoluble group into an alkali soluble group by Post Exposure Bake (PEB) using the generated acid as a reaction catalyst and removing the exposed area by alkali development.
In the liquid immersion exposure, a resist film is exposed through a photomask in a state where a space between the resist film and the optical lens is filled with the dipping solution (also referred to as a liquid immersion solution) to transfer the pattern of the photomask to the resist film. However, it is anticipated that the dipping solution permeates inside the resist film thereby giving undesired effect on the performance of the resist.
When a chemical amplification resist is applied to the liquid immersion exposure technique, the acid on the resist surface generated during exposure is transferred to the liquid immersion solution to change the concentration of the acid at the surface of the exposed area. It is considered that this is extremely similar with acid deactivation on the surface of the exposure area caused by basic contamination of an extremely small amount at several ppb level from the circumstance during Post Exposure time Delay (PED) which caused a significant problem in the initial stage of the development of the chemical amplification type positive resist but the effect and the mechanism given by the liquid immersion exposure on the resistor have not yet been apparent.
On the other hand, in a case of applying chemical amplification type resist having no problem in the lithography by usual exposure to the pattern formation by the liquid immersion exposure, it has been found problems that development defects and development residues (scums) are formed, or leaching of the resist to the liquid immersion solution occurs.